The structure, electronics, and performance characteristics of the common loudspeaker are well documented in the following texts and anthologies: Acoustical Engineering, Harry F. Olson, Ph.D., Professional Audio Journals, Inc., Philadelphia, Pa. (1991, Library of Congress Catalog Card No. 91-075297); Acoustics Leo Beranek, American Institute of Physics, New York, N.Y. (1986, Library of Congress Catalog Card No. 86-70671); Loudspeakers, An anthology of articles on loudspeakers from the pages of the Journal of the Audio Engineering Society Vol. 1-Vol. 25 (1953-1977), 2nd Edition, Audio Engineering Society, Inc., New York, N.Y. (1980, Library of Congress Catalog Card No. 80-53465)(referred to below as "Anthology I"); and Loudspeakers, An anthology of articles on loudspeakers from the pages of the journal of the Audio Engineering Society Vol. 26-Vol. 31 (1978-1983), Audio Engineering Society, Inc., New York, N.Y. (1984, Library of Congress Catalog Card No. 78-61479)(referred to below as "Anthology II"), each of which is incorporated herein by reference.
As discussed throughout the above-identified literature, the conical diaphragm is one of the most common forms of loudspeakers and is typically manufactured of fabric or plastic. It is generally considered the weakest link in the audio reproduction system.
More specifically, the audible sound spectrum contains widely different frequencies in the range of about 16 Hz to 20,000 Hz, and when alternating currents of those frequencies are applied to the common conical loudspeaker, the diaphragm will vibrate in different modes of lower and higher order. At lower frequencies, the conical diaphragm vibrates as relatively rigid body, and correspondingly, distortion remains low. However, the common conical diaphragm is not rigid enough to withstand the inertia forces that occur at higher frequencies. As a result, when higher frequency audio signals are applied to the common conical diaphragm, it starts to vibrate not as one unit, but in parts, causing correspondingly increased distortion in reproduced sound. See "Vibration Patterns and Radiation Behavior of Loudspeaker Cones," F. J. M. Frankfort, reproduced in Anthology II at pp. 16-29, and "Computerized Analysis and Observation of the Vibration Modes of a Loudspeaker Cone," reproduced in Anthology II at pp. 301-309, for a more detailed discussion of those drawbacks.
Many design efforts have focused on increasing the rigidity of the common conical loudspeaker diaphragm. In that regard, it is known that the most desirable characteristics of materials used for the loudspeaker diaphragm are high modulus E, low density p, moderate internal loss and low overall weight. A large value of the ratio E/p is desirable to extend the high frequency limit and to reduce harmonic distortion.
In one application, boronized titanium conical diaphragms were reportedly formed. See "High Fidelity Loudspeakers with Boronized Titanium Diaphragms," reproduced in Anthology II at p. 198-203. In a second approach, a polymer-graphite composite sheet was reportedly formed using graphite crystallite granules with polymer additives. The composite sheet was formed into various shapes for either low-frequency or high-frequency loudspeakers. See "Polymer-Graphite Composite Loudspeaker Diaphragm," reproduced at Anthology II at pp. 272-277.
It a third design, conical diaphragms were molded from olefin polymers and carbon fibers which were mixed together, treated and formed into a paper, which was then heated. In accordance with this approach, for larger diaphragms, the reinforced polymer material was applied as a sandwich structure, having the reinforced polymer sheets as the two surface materials, and an organic foaming sheet as the core. See "Reinforced Olefin Polymer Diaphragm for Loudspeakers," reproduced in Anthology II, at pp. 286-291. In a fourth application, conical loudspeakers were formed of sandwich construction consisting of aluminum outer skins with expanded polystyrene cores. See "The Development of a Sandwich-Construction Loudspeaker System," reproduced in Anthology I, at pp. 159-171. In this last article, it is stated that honeycomb aluminum or impregnated paper are frequently used as cores for sandwich construction in aircraft applications and could be used for flat diaphragms, but that the conical design was preferred because of increased rigidity.
However, it is known that the common conical loudspeaker design, which was adopted due to its increased rigidity as compared to other shape diaphragms, has additional drawbacks. Most importantly, a small apex angle for a conical diaphragm is necessary to achieve high resonance frequencies. However, a small apex angle also results in peaks and dips in the loudspeaker's frequency response. This problem has been addressed to some degree by using several conical loudspeakers of different diameters to cover the sound spectrum in multi-channel loudspeaker systems. However the problem still remains that the arrival times of sounds from the different conical loudspeakers vary depending on the number and relative apex angles of the different loudspeakers. Accordingly, in a fifth design approach, a coaxial flat-plane diaphragm was fabricated using a sandwich-type construction consisting of two polymer-composite sheets with an aluminum foil honeycomb core bonded in between. See "Coaxial Flat-Plane Loudspeaker with Polymer Graphite Honeycomb Sandwich Plate Diaphragm," reproduced in Anthology II, at pp. 278-285. In a sixth application, a honeycomb disk diaphragm is driven at the first nodal line of its resident mode, and is constructed using honeycomb sandwich plates in which the honeycomb core is axially symmetrical with a cell density distribution that increases toward the center of where the bending stress is most concentrated. See "Loudspeaker with Honeycomb Disk Diaphragm," reproduced in Anthology II, at pp. 263-271. In this last application, the sandwich disk is made entirely of aluminum foil.
In each of the above applications, either the construction techniques were difficult or expensive, making them impractical for efficient, large-scale commercial manufacture, the resulting diaphragm was relatively heavy, resulting in decreased performance, or the modulus to density ratio (E/p) was still too low, requiring the diaphragm to be driven at the first node of vibration, thereby further complicating manufacture. In addition, many of the designs continue to employ conical loudspeakers, which exhibit the "cavity effect" described above. Thus, the need still exists for an improved flat plane diaphragm having the desirable characteristics of high modulus E, low density p, moderate internal loss and low overall weight, and which is easily and efficiently mass produced at relatively low cost.
The preferred embodiments of the inventions are described below in the Figures and Detailed Description. Unless specifically noted, it is intended that the words and phrases in the specification and claims be given the ordinary and accustomed meaning to those of ordinary skill in the applicable art or arts. If any other meaning is intended, the specification will specifically state that a special meaning is being applied to a word or phrase.
Likewise, the use of the word "function" in the specification is not intended to invoke the provisions of 35 U.S.C. .sctn. 112, .paragraph.6 to define the invention. To the contrary, that paragraph will be considered to define a claimed element of the invention, only if the phrases "means for" or "step for" and a function, without also reciting in that element any structure, material, or act in support of the function, are specifically recited in that element. Moreover, even if the provisions of 35 U.S.C. .sctn. 112, .paragraph.6 are invoked to define the invention, patentee intends that the invention not be limited to the specific structure, material, or acts that are described in the preferred embodiments. Rather, "means for" or "step for" elements are nonetheless intended to cover and include within their scope any and all known or later-developed structures, materials, or acts that perform the claimed function, along with any and all equivalents.